Presently, DPP solutions predominantly consist of a fine pattern of printed optical marks over the whole surface of the paper that when imaged by a miniature camera near the tip of a pen or other electronic pointing device provides absolute position and page information. An example of such a system is given in U.S. Pat. No. 6,548,768 to Anoto A B. According to U.S. Pat. No. 6,548,768, a position code is defined by the combination of the position of a visible dot in relation to an invisible virtual grid vertex reference point. The layout of the virtual grid is defined by the overall arrangement of the visible dots and is deduced by imaging a number of dot locations. The actual position coding data is decoded by determining the position of the visible dot in relation to its virtual grid vertex reference point. For four dot positions around the grid vertex, four unique values can be encoded in any one grid position.
An alternative position coding pattern has been developed by the current applicant and is the subject of International patent application WO 2005/043448. This document discloses an optical position code comprising a first reference grid of marks that provides the position anchors to then detect within the four corners one of nine possible positions of a data mark, thereby defining an alphabet of nine symbols corresponding to the different dot displacements on the inner grid. This pattern is then repeated across the surface of the paper.
In the predominant digital pen and paper systems the electronic pen or pointing device includes an imaging system, generally an optical camera system, that captures a defined field of view that encompasses a portion of the optical position code, from which the location of the electronic pen can be calculated. It will therefore be appreciated that the angle at which the electronic pen or pointer is held with respect to the plane of paper can not only vary from user to user and also vary for an individual user depending upon particular style of handwriting or use, but more importantly is rarely, if ever, orientated perpendicular to the plane of the paper on which the optical position code is printed. Consequently, the image captured by the electronic pen and its associated imaging system invariably has an element of perspective distortion associated with it. Determining the location of the optical data marks with reference to either a virtual or actual reference grid in such a distorted image can be difficult.